The present invention relates to a process for continuously producing chlorine dioxide from an alkali metal chlorate sulfuric acid or another chlorine free mineral acid and hydrogen peroxide as reducing agent at a pressure from about 400 mm Hg to about 900 mm Hg, preferably at atmospheric pressure. In particular the invention provides an efficient process with high yield of chlorine dioxide and with essentially no chlorine byproduct.
Chlorine dioxide in aqueous solution is of considerable commercial interest, mainly in pulp bleaching, but also in water purification, fat bleaching, removal of phenols from industrial wastes etc. It is therefore desirable to provide processes in which chlorine dioxide can be efficiently produced- Considerable research is also directed to the handling of by-products such as chlorine and mineral acid salts.
A number of different processes for producing chlorine dioxide are known in the art. Several processes use the same raw materials and produce the same sort of residual products at the same reaction conditions. The only difference is the reducing agent.
Alkali metal chlorate and sulfuric acid is brought continuously to a reaction vessel to which air and the reducing agent are introduced, usually into the bottom of the vessel. Then chlorine dioxide and air leave from the top of the reaction vessel and a depleted reaction solution is withdrawn for further treatment. It is common to use more than one vessel whereby the depleted reaction solution from the first vessel is brought to a second (and further) vessel together with air and reducing agent for further conversion of the remaining chlorate. The reaction in the reaction vessel/s is carried out at about atmospheric pressure. Reducing agents used in this type of reaction are sulfur dioxide (the Mathieson process), methanol (the Solvay process) and chloride ions (the R-2 process). The basic chemical reaction involved in the process with chloride ions can be summarized with the formula: EQU ClO.sub.3 +Cl+2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O[1].
The other reducing agents are indirect reducing agents, the direct reaction between chlorate ions and methanol or sulfur dioxide being very slow. The direct reducing agent in these cases are chloride ions reacting according to [1]. The chlorine produced is then reacting with methanol to regenerate chloride ions according to the formula: EQU CH.sub.3 OH+3Cl.sub.2 +H.sub.2 O.fwdarw.6 Cl+CO.sub.2 +6H.sup.+ [2]
or with sulfur dioxide according to the formula: EQU SO.sub.2 +Cl.sub.2 +2 H.sub.2 O.fwdarw.2 HCl+H.sub.2 SO.sub.4[ 3].
As is evident from [1] a large amount of chlorine is produced as a by-product when chloride ions are used as reducing agent. To reduce the amount of chlorine by-product formed in the process, methanol was used instead of chloride ions as the reducing agent. However, also with methanol and with sulfur dioxide a certain amount of chlorine is produced as chloride ions are involved in the reduction process. It is also common in these processes to add a small amount of chloride ions, in the form of sodium chloride or hydrochloric acid to increase the efficiency. Formerly the chlorine by product has been utilized in the paper mills but due to increased environmental demands there is a decreasing need for chlorine.
The change over from chloride ions to methanol as the reducing agent also resulted in the disadvantage of formation of other by-products than chlorine in the reaction system. The reaction according to formula [2] above does only represent the theoretical methanol oxidation. However, in practical production inefficiencies in the methanol oxidation bring about the formation of formaldehyde and formic acid and probably also ethers and esters along with the carbon dioxide. It could be expected that reactions can occur in the bleaching train with these by-products resulting in chlorinated organic compounds.
Besides the draw back with formation of chlorine and other by-products, the old R-2, Solvay and Mathieson processes also have the disadvantage of low efficiency and low production rates. The efficiency for a normal Mathieson process calculated as chlorate transformed into chlorine dioxide is typically not more than about 88%.
To increase the efficiency of these processes it was suggested to run the processes in a single vessel under sub-atmospheric pressure. Chlorine dioxide is then generated continuously together with the evaporated aqueous reaction medium. The alkali metal sulfate by-product is crystallized. This process is disclosed e.g. in U.S. Pat. No. 4,081,520. This process and similar "single vessel process" ("SVP" processes) technologies generally increase the efficiency to acceptable levels while maintaining low levels of chlorine effluent. Patents issued after the above mentioned describe different embodiments attempting to optimize the process with as low chlorine production as possible.
Another reducing agent suggested in the state of the art for chlorine dioxide production is hydrogen peroxide. U.S Pat. No. 2,332,181 discloses a batch process for chlorine dioxide production of substantially pure chlorine dioxide with respect to chlorine with hydrogen peroxide as the reducing agent. The process must be run at a low temperature and with low concentrations in the reactor to avoid explosive decomposition. Other patents suggest a combination of hydrogen peroxide and chloride ions as the reducing agent. This combination has the disadvantage of chlorine formation. In U.S. Pat. No. 5,091,167 the applicant found that it was possible to produce chlorine dioxide continuously with high efficiency with hydrogen peroxide as the reducing agent in a chlorine free process with the SVP technology.
However, there is still a need for developing chlorine dioxide processes at atmospheric pressure with good efficiency and production rate but with reduced production of chlorine byproduct as well as other by-products. For example there are a large number of existing plants with atmospheric pressure generators with poor efficiency and with capacity limitations. With increasing demand for chlorine dioxide bleaching, improvements of these plants would be of considerable interest. Also for the installation of new plants the atmospheric pressure process offers a low investment cost for the chlorine dioxide generator.